As a master plate sheet clamping device for mounting a thin master plate sheet such as a stencil master plate sheet on the outer circumferential surface of a printing drum of a rotary stencil printing device, there has been proposed, for instance in Japanese utility model publication (kokoku) No. 1-26463, a master plate sheet clamping device comprising a clamping plate which can angularly move approximately by 180 degrees between a clamping position and an unclamping position around an axial line of the printing drum, and clamps a master plate sheet between the clamping plate at its clamping position and the outer circumferential surface of the printing drum.
The angular movement of the clamping plate between the clamping position and the unclamping position in such a clamping device is accomplished by a gear mounted on a support shaft of the clamping plate, and a motor having an output shaft carrying a drive gear, the motor being moveable between a meshing position for meshing the drive gear with the gear on the support shaft, and a release position for disengaging the drive gear from the gear on the support shaft so as to allow the printing drum to rotate freely. Thus, by moving the motor to the meshing position and bringing the drive gear on the output shaft of the motor in engagement with the gear on the support shaft, the support shaft on the printing drum is angularly actuated by the motor outside the printing drum.
According to such a master plate sheet clamping device, a relatively large solenoid device is necessary for moving the motor between the meshing position and the release position, and there is a possibility that the drive gear on the motor output shaft may not properly mesh with the gear on the support shaft, and the clamping plate may not be properly actuated even when the motor is moved to the meshing position.
Furthermore, to angularly actuate the support shaft on the printing drum from outside the printing drum, a certain mechanism is necessary to prevent the rotation of the printing drum when angularly actuating the support shaft so that the printing drum may be kept stationary and a necessary reaction force may be obtained, and this necessarily increases the complexity of the mechanism.